Lignin is derived from wood as a by-product in the pulping process and is among the most abundant, renewable, natural products on earth. Tremendous quantities of lignin are produced each year that are generally disposed of as waste materials. Relatively small amounts of lignin are used commercially as dispersing and grinding agents, sequestering agents, in vanillin production and in reactions for the production of sulfur-containing compounds.
Lignin materials have found limited utilization commercially because of the complexity, chemically and physically, of lignin. In spite of extensive research in the art over a long period, there remins a great deal of uncertainty as to its properties and reactions. As it occurs in natural lignocellulose, lignin is a complex substance in the nature of a nonuniform polymeric structure in which the basic molecular configuration is believed to be derived from repeating propyl phenol units. The exact chemical structure of lignin is not known although the presence of ether linkages within the structure and the presence of benzene rings, methoxy groups and both aliphatic and phenolic hydroxyl groups is well established. Kraft lignin is known to be a polymer of substituted catechol with more than half of the potentially reactive aromatic hydroxyl groups being blocked by methyl groups. If such methyl groups could be removed by an economically feasible method, the resulting material should be highly suitable for subsequent reaction to produce useful products. However, like common aromatic ether cleavages, the demethylated material is generally either too reactive to survive the severe reaction conditions necessary for demethylation or it is converted into a different material under the reaction conditions. Similarly, demethylation of lignosulfonates from spent sulfite liquors in general results in materials that are so unstable in the reaction mixture that either substantial decomposition occurs or the polymer is converted into a different material.
It is known that lignin materials may be treated to cleave the methoxy groups. For example, U.S. Pat. No. 2,816,832 produces dimethyl sulfide from Kraft black liquor by autoclaving at 250.degree. C. under pressure of 680 to 820 psi in the presence of elemental sulfur; U.S. Pat. No. 2,840,614 produces methyl mercaptan by treating lignin-containing solutions with inorganic sulfur materials capable of reacting with the methoxy groups of the lignin in an autoclave at 215.degree.-220.degree. C.; U.S. Pat. No. 2,908,716 treats spent liquors from alkaline pulping with methyl mercaptan to cleave the alkoxy group to form sulfides at 240.degree. C. under autoclave pressure and U.S. Pat. No. 3,326,980 produces methyl mercaptan by pyrolysis of spent pulping liquors, i.e., by heating to decompose the lignin material in a furnace at 300.degree. C. for 2 hours. Another method well known in the art as the Ziesel method, is an analytical tool for determining methoxyl content. In such method, lignin is demethylated by heating with HI at 150.degree. C. and recovering methyl iodide. This method is a quantitative analytical tool rather than a preparation method. In this process, the demethylated lignin is neither isolated nor studied because of the strong acidic conditions. Each of the methods referred to above demethylates lignin to some extent. However, to my knowledge, no attempts have been made to preserve the lignin polymer backbone nor to recover the demethylated lignin as such, demethylation in these cases being merely incidental either to recovery of a sulfur-containing product or to the determination of methoxy content.
It has been reported in the literature by G. F. Zakis et al in Khimiya Drevesiny, 14, p. 98 (1973) that hydrolysis lignin may be demethylated by reaction with pyridinium hydrocholoride at 180.degree. C. for 3 hours according to the general equation: ##STR1##